Behavior and failure mode of reinforced concrete members damaged by pre-cracking

Transcription

1 ORIGINAL ARTICLE Behavior and failure mode of reinforced concrete members damaged by pre-cracking Amorn Pimanmas 1 Abstract damaged by pre-cracking Songklanakarin J. Sci. Technol., 2007, 29(4) : The effect of pre-cracking on the behavior and failure mode of reinforced concrete beams damaged by pre-cracking is experimentally studied in this paper. The control beam was designed to fail in ductile flexural yielding under four-point bending and in brittle shear under three-point bending. The effect of precracking is studied under both four-point bending and three point bending. In the former, pre-cracks are inclined with respect to the beam axis and the shear span is short, hence the external load is resisted by diagonal compression strut. In the latter, pre-cracks are orthogonal to the beam axis and the shear span is moderately long, hence the external force is transferred through concrete tensile strength, i.e., shear in moderately slender beam. The tests under these two load conditions therefore cover the effect of pre-cracking on concrete under compression and tension where the mode of load resistance is different. It is shown that when the shear span is short, pre-crack reduces the shear crushing capacity due to the reduction in effective contact area and compressive strength deterioration due to micro-fracturing damages. The presence of precracks can change the failure mode from ductile flexure to brittle shear. On the other hand, when the shear span is longer, pre-crack elevates the shear capacity through crack arrest mechanism. In both cases, the precracking is demonstrated to be structurally significant and should be properly taken into account when analyzing existing members. Key words : pre-crack, load history, crack arrest, shear crushing capacity, failure mode 1 Ph.D. (Civil Engineering), Assoc. Prof., School of Civil Engineering and Technology, Sirindhorn International Institute of Technology, P.O. Box 22, Thammasat Rangsit Post Office, Pathum Thani, Thailand amorn@siit.tu.ac.th Received, 10 July 2006 Accepted, 8 January 2007

2 Vol. 29 No. 4 Jul. - Aug àõ Õ æ π» 1 惵 Ÿª «µ ÕßÕß åõ Õπ µ À Á Ë â «À Õ â «à«ßàπâ «. ß π π å « (4) : «π È πõ Õß æ ËÕ» º Õß Õ â ß à«ßàπâ µàõ惵 Ÿª «µ Õß π Õπ µ À Á Èπ Õ «ÿ Ÿ ÕÕ Àâ «µ Àπ «µâπè Àπ ß 4 ÿ Àâ «µ Õπ ª µâπè Àπ ß 3 ÿ» º Õß Õ â «à«ßàπâ â Èß µâπè Àπ ß 4 ÿ 3 ÿ µâπè Àπ ß 4 ÿ π Èπ Õ â «à«ßàπâ» ÿ Õ ß π π π à«ß Õπ Ë Èπ Ëß Àâ µâ π π ß πõ Õß ππ Èπ â Õß àõπ ßÕ ß à«π µâπè Àπ ß 3 ÿ π Èπ Õ â «à«ßàπ⻵ Èß π π π à«ß Õπ Ë «ª π ß Ëß Àâ µâ π π ß Õππ ÈπÕ» ß ß Õß Õπ µ ªìπÀ ßπ Èπ Õ Èß Õß π È Õ ÿ» º Õß Õ â «à«ßàπâ Èß π æ Ë Õπ µ ßÕ ß ß º Õ ß Àâ ÀÁπ«à π Ë π à«ß Õπ Èπ Õ â «à«ßàπâ Àâ ßµâ π π ß ÕπÕ ß π ËÕß Õ â «à«ßàπâ Àâæ Èπ Ë º ª º Õß Õπ µ ß Àâ ß ßÕ Õß Õπ µ «â Õ â «µë ß π ËÕß π ÈÕ Õπ µ «Õ â ««À Õ µ π ÿ πõ π È Ÿª «µ Á ª Ë π ª ß «µ À𠫪ìπ Õπ ª π ßµ ß π â π Ë π à«ß Õ𠫪 π ß Õ â «à«ßàπâ º Àâ ßµâ π π ß Õπ Õß π Ÿß Èπ π ËÕß À ÿ Õ â «π Èß Õß π Èπ æ «à Õ â «à«ßàπâ º µàõ惵 ß â ß Õß π ªìπÕ à ß ßπ Èπ π «Àå Õß åõ Ë Õ Ÿà π Èπ µâõßæ º Õß Õ â «à«ßàπâ â«π π π π µ π µÿâ ª. 22»Ÿπ å ª å ß µ ßÀ«ª ÿ π Existing reinforced concrete structures in current service may experience various load events throughout their service life. For example, RC structures in seismic zones may experience earthquake loading that causes pre-cracks in beams, columns and other components. The function of concrete structure may also change from time to time. Concrete is a path-dependent material, hence, the past loading may create cracking, residual stress and strain in the members. This paper attempts to investigate the effect of past load history, especially existing cracks resulting from previous load stages, on the structural behavior at ultimate state in the next load stages. The past load history is not only limited to external forces but also includes environmental actions. Shrinkage and temperature cracks are examples of such environmental effects. In the past, the effect of pre-cracking was mainly studied in terms of serviceability of reinforced concrete members. The primary concern for excessive cracking is of serviceability matter including appearance, leakage and corrosion. To the author's knowledge, the effect of pre-cracking on the ultimate capacity has not been discussed or fully understood (Pornpongsaroj and Pimanmas, 2002; Yamada et al., 1995). Previous researches investigated the shear capacity of RC members subjected to axial tension (Tamura et al., 1991, 1999). In their works, axial tension was applied first, maintained on the beam and then shear was superimposed. The first state axial tension may simultaneously induce pre-cracks as well as prestress inside the beam. In this case, it is hardly possible to differentiate the sole influence of precrack from the pre-stress state. Unlike previous investigation, this paper aims to experimentally

3 Vol. 29 No. 4 Jul. - Aug examine the sole influence of pre-cracks in terms of crack interaction. In the experimental program, the axial tensile stress is not introduced into the beam. Only pre-cracks are induced. The author believes that the study of reinforced concrete members experiencing past load history would be of prime importance for the evaluation of the structural performance of existing structures. This paper presents an experimental research concerning the structural effect of precracking on the ultimate limit state of reinforced concrete members. The primary concern is the ultimate shear capacity of RC members with initial pre-cracks. Significance of Research During the service life of concrete structures, they may be subjected to various loading types and environmental conditions. The crack systems generated by different load effects may be unrelated. The cracks caused by previous load effects are initial damages that may structurally affect the behavior in the next loading events. Most current design methods aim at new structures. Specifically, they are suitable to undamaged reinforced concrete members. The study of precracking effect on structural behavior is needed for the evaluation of existing structures. This paper experimentally investigates the effect of pre-cracking and demonstrates its structural importance. Experimental program 1. Specimen Design, Beam Dimensions and Reinforcement Detailing The dimension and reinforcement detailing of the tested beams are shown in Figure 1. Two beams, B1 and B2, with identical cross sectional dimension and reinforcement detailing were tested. The bottom longitudinal reinforcement consisted of 2DB25 (25-mm deformed bar) with the area A s of 982 mm 2. The top longitudinal reinforcement consisted of 2DB25 (25-mm deformed bar) with the area A s of 982 mm 2. The longitudinal reinforcement ratio was 1.95%. The stirrups consisted of RB6 (6-mm plain bar) spaced at 120 mm center to center in the 1200 mm central portion of the span and RB9 (9-mm plain bar) spaced at 70 mm center to center in each 500 mm end as shown in Figure 1. The effective depth was mm. The clear concrete cover was 30 mm. The beam dimension and reinforcement detailing were designed such that the effect of pre-cracking on flexural and shear failure mode of the beam could be investigated within the same specimen. 2. Loading method to examine the effect of precracking under four-point bending The loading arrangement to examine the effect of pre-cracking under four-point bending is shown in Figure 2(a). The beam was tested with the shear span to effective depth ratio of 2.0 which is considered as short shear span. The 1200-mm central part was subject to pure moment whereas each 500-mm end was subject to shear and moment. Under this load condition, the external load is transferred directly to the support through diagonal compression strut. The inclined cracks were generated in the end zones as shown in Figure 2(a). In the first stage, since the yielding capacity was less than the shear failure load, yielding took place before shear failure. In the second stage, the beam was rotated 180 degree about its axis. It was Figure 1. Beam dimension and reinforcement detailing

4 Vol. 29 No. 4 Jul. - Aug Figure 2. Load stages in the experiment loaded again under four-point bending with the pre-cracking condition resulting from the first stage loading. The target of interest was the effect of inclined pre-cracking at the end zones on the shear capacity. 3. Loading method to examine the effect of pre-cracking under three-point bending The first and second stage four-point bending also generated vertical pre-cracks in the mm central part of the member. The effect of these vertical pre-cracks on the shear response of the member was studied by applying three-point bending to the beam as shown in Figure 2(b). This was considered as the 3 rd load stage. The ratio of shear span to effective depth was 2.40, which is moderately long. Under this load condition, inclined crack formed in the shear span. The effect of vertical pre-cracking on formation and propagation of the inclined crack was the target of study. After the beam was tested under the third stage of three-point bending, it was rotated by 180 degree and loaded again in three-point bending in the 4 th load stage. Under the 4 th load stage, the pre-cracking condition consisted of penetrating vertical precracking resulted from four-point bending and inclined pre-cracking resulted from the three-point bending. The shear behavior under multi-precracking condition was examined in the 4 th load stage. 4. Material properties and sectional capacities Ready mixed concrete supplied from local plant was used to cast specimens. The average tested compressive strength of standard mm cylindrical concrete specimens at 28 days was 29.6 MPa. The tested yield strengths of RB6, RB9 and DB25 were 339.4, and MPa, respectively. The tested ultimate strengths of RB6, RB9 and DB25 were 467.7, and MPa, respectively. These material properties are summarized in Table 1. Based on the tested material properties and geometry of the beam specimens, the flexural capacity was calculated using the beam's sectional analysis. The concrete shear capacity was calculated using the modified Okamura-Higai equation (Okamura and Higai, 1980), which was a predictive equation rather than a conservative code equation. The stirrup shear capacity was calculated using equilibrium equation assuming yielding in all transverse stirrups cut by inclined cracks. Table 2 summarizes the shear and flexural capacities of the cross section.

5 Vol. 29 No. 4 Jul. - Aug Experimental results and Discussions Table 3 summarizes the experimental loading capacities and failure modes of the tested beams. The discussion of the behavior will be given in the following sections. The behavior of beams B1 and B2 were generally similar to each other in all load stages. B1 was tested under four load Table 1. Material properties Materials Yield strength Ultimate strength (MPa) (MPa) Steel DB Steel RB Steel RB Concrete Tested compressive strength ( f c ) = 29.6 MPa stages as shown in Table 3. But B2 was tested under three load stages only because the cracking condition and deformed shape of B2 after the 3 rd stage were so severe that it was not appropriate to conduct the 4 th load stage on this beam. 1. Behavior of beam under four-point bending The load versus mid-span deflection of B1 and B2 under four-point bending is shown in Figure 3. In the first stage of 4-point bending, flexural cracks were created in the central portion of the beam while diagonal cracks were created in the end portions (see Figure 5(a)). The diagonal shear cracks were observed to be inactive due to the presence of large number of stirrups. On the other hand, flexural cracks grew in size continuously corresponding with yielding of steel bars. The load versus mid-span deflection showed ductile flexural yielding (Figure 3) as designed. In the second load Table 2. Sectional capacities Sectional Properties Four-point bending Three-point bending Shear span / effective depth (a/d) Concrete shear capacity (V c ), kn 78.9 (Okamura H. and Higai T., 1980) Stirrup shear capacity (V s ), kn Total shear capacity (V c +V s ), kn V s / (V c + V s ) Shear failure load, kn Flexural moment capacity, kn.m Yielding load, kn Table 3. Summary of loading capacity and failure mode Beam B1 Beam B2 Loading Ultimate Load, Failure mode Ultimate Load, Failure mode kn kn Four-point bending Flexural yielding Flexural yielding (0 o ) 1 st stage Four-point bending Shear failure Shear failure (180 o ) 2 nd stage Three-point bending Shear failure Shear failure (0 o ) 3 rd stage Three-point bending Shear failure - - (180 o ) 4 th stage

6 Vol. 29 No. 4 Jul. - Aug Figure 3. Load versus mid-span deflection of tested beam under four-point bending Figure 4. Load versus mid-span deflection of tested beam under three-point bending stage of 4-point bending, the beam failed in brittle shear rather than flexural yielding. A large shear crack suddenly formed while the growth of the flexural crack was not active (Figure 5(b)). The maximum load attained was kn compared with kn (yielding load) in the first load stage. This experiment showed that the existing pre-cracking could change the failure mode of the beam from ductile flexure to brittle shear. In the first load stage, the yielding load was designed to be lower than the shear failure load (see Table 3). Due to pre-cracking, however, the yielding load became higher than the shear failure load in the 2nd stage. This indicated that the pre-cracking could significantly affect the original design condition. It was supposed that pre-cracking would bring about the brittleness to the beam. The source of brittleness was derived from the reduction in compressive strength of pre-cracked concrete. The web concrete in the end portion of the beam was subjected to a large diagonal compressive force as a result of the transfer of an external load to the support through two main mechanisms: direct compression strut and truss mechanism. The inclined pre-cracking created in the 1 st load stage was roughly orthogonal to the diagonal compression in the 2 nd load stage. Owing to pre-crack, the contact area through which the compressive force could be transferred was apparently reduced. Additionally, it is supposed that the compressive strength of concrete close to the pre-crack was degraded because of micro-fracturing damages (Maekawa et al., 2003). These two mechanisms are explained in Figure 6. As a result, the compressive capacity of concrete was reduced, leading to the decrease in shear capacity according to strut and truss mechanism.

7 Vol. 29 No. 4 Jul. - Aug Figure 5. Crack pattern observed in each loading stage for a typical beam (Color figure can be viewed in the electronic version) Figure 6. Compressive strength degradation mechanisms of pre-cracked concrete

8 Vol. 29 No. 4 Jul. - Aug Behavior of beam under three-point bending The load versus mid-span deflection of B1 and B2 under four-point bending is shown in Figure 4. After the beam was tested under four-point bending, the vertical pre-cracking damage was induced in the beam. The average crack width caused by four point bending was observed to 1.8 and 1.6 mm for beam B1 and B2, respectively. The beam was then tested under three-point bending. The target zone of study was the 1200-mm central span with penetrating vertical pre-cracking (Figure 2(b)) created in the 1 st and 2 nd load stages. Under three-point bending, the beam was designed to have the shear failure load lower than the yielding load (see Table 3) in order to study the effect of pre-cracking on the shear response. Beam B2 was loaded to failure in shear with the loading capacity of kn in the 3 rd load stage. The failure crack pattern of B2 under the 3 rd load stage is shown in Figure 5(c). It was noted the beam could achieve much larger shear capacity compared with calculated values, which ranged from to 262 kn. The former was computed using tested yield strength and the latter using tested ultimate strength of stirrups. Since the calculation of stirrup shear capacities was based on tested strengths, they should be very close to the actual values. The increase in total shear capacity must therefore be attributed to the increase in concrete shear capacity. The simple calculation showed that the increase in concrete shear capacity ranged from 43% (based on tested ultimate strength of stirrup) to 53% (based on tested yield strength of stirrup) from the Okamura-Higai, 1980 shear capacity equation. It should be noted that the Okamura-Higai equation was the predictive equation, not a design equation. Hence, this equation should predict the shear capacity close to reality. In fact, the ACI design equation (ACI318-05) would predict the concrete shear capacity to be only kn compared with 78.9 kn computed by the Okamura- Higai equation. The increase in concrete shear capacity was explained (Pimanmas et al., 2001) to be the consequence of the crack arrest phenomenon. The mechanism of the crack arrest is illustrated in Figure 7. A beam with a pre-crack located approximately at the center of shear span is shown. Since principal stresses that can develop in the element close to pre-crack are low, the diagonal crack can hardly form in the element. As a result, the diagonal crack is apparently arrested there. This results in the increase in concrete shear capacity. As shown in Table 3, the ratio of stirrup shear capacity was 0.66 under four-point bending and 0.36 under three-point bending. A large ratio of stirrups would put a higher demand on concrete compression. As explained, the majority of shear capacity was derived from concrete compression in the four-point bending. In contrast, under the three-point bending, the shear capacity was partly contributed by concrete tension (V c ) and partly by concrete compression associated with the truss mechanism. The concrete tension is shear capacity against sliding after the formation of a complete inclined crack that links the load point to the support. As a crack is arrested at the pre-crack Figure 7. Mechanism of crack arrest

9 Vol. 29 No. 4 Jul. - Aug Figure 8. Crack arrest phenomenon observed in the experiment Figure 9. Crack arrest phenomenon observed by Pimanmas (Pimanmas et al., 2001) (Color figure can be viewed in the electronic version) plane, the formation of a complete crack is inhibited, resulting in the increase in concrete shear capacity. Since there were fewer stirrups in the central 1200-mm portion, the truss mechanism in which concrete was subjected to compression was not dominant and truss shear capacity was mainly governed by the yielding of stirrups. For B2, the pre-cracking elevated the loading capacity up to kn, almost reaching the yielding load of kn (Table 2). Pimanmas et al., 2001 experimentally showed that the beam designed to fail in shear could actually turn to fail in ductile flexural yielding due to the effect of pre-cracking. The crack arrest phenomenon demonstrated in this experiment and that observed by Pimanmas et al., 2001 and shown in Figures 8 and 9, respectively. As for B1, the beam was not loaded to failure in the 3 rd load stage of three-point bending, but to 271 kn to create inclined cracks in the central portion. This was to create a multi-directional cracking state that consisted of vertical pre-crack- ing and inclined pre-cracking before the beam was tested in the 4 th stage of three-point bending. Despite the severe pre-cracking condition, the beam could achieve very high loading capacity, well above the calculated value shown in Table 3. The crack arrest and diversion phenomenon is supposed to be responsible for the increase in concrete shear capacity. The failure condition of beam B1 under the 4 th load stage is shown in Figure 5(d). Conclusions The significance of pre-cracking was experimentally demonstrated in this paper. The failure modes could be changed from ductile flexure to brittle shear and vice versa. In the four-point bending, the effect of pre-cracking on concrete subjected largely to compressive force was investigated. In the three-point bending, the effect of pre-cracking on concrete subjected largely to

10 Vol. 29 No. 4 Jul. - Aug tensile force was investigated. On compression, pre-cracking reduced the compressive strength of concrete by reducing the effective contact area and compressive strength deterioration due to microfracturing damages. This leads to a drop in web crushing shear capacity. On tension, pre-cracking arrested the propagation of the diagonal crack, leading to a rise in concrete shear capacity. The effect of pre-cracking should be properly taken into account in the evaluation of the safety of precracked concrete structures. Acknowledgements The authors are very grateful to Thailand Research Fund for providing the research fund TRG to carry out the research. References ACI committee Building code requirements for structural concrete (318M-05) and commentary (318RM-05). Farmington Hills, Michigan. Maekawa K., Pimanmas A. and Okamura H Nonlinear mechanics of reinforced concrete, Spon Press, London Okamura H. and Higai T Proposed design equation for shear strength of reinforced concrete beams without web reinforcement. Proceeding of JSCE, 300: Pimanmas A., and Maekawa K Influence of precracking on reinforced concrete behavior in shear. Concrete Library International, JSCE, No.38: Pornpongsaroj P. and Pimanmas A Change of failure mode of RC beam damaged by preloading. Proceeding of the 8 th National Civil Engineering Convention, Konkaen, Thailand (in Thai). Tamura T., Shigematsu T. and Nakashiki K About the increase in shear capacity of RC beam under axial tension. Proceeding of the 54 th JSCE annual convention, (In Japanese) Tamura T., Shigematsu T., Hara S. and Nakano S Experimental analysis of shear strength of reinforced concrete beams subjected to axial tension, Concrete Research and Technology, JCI, 2(2). Yamada K. and Kiyomiya O., Shear resistance of reinforced concrete beams with initial penetrating cracks, Proceeding of JCI, 17(2): (In Japanese).